X-ray imaging technology is becoming increasingly popular. The corresponding X-ray or CT imaging apparatus generally includes an X-ray tube and an X-ray detector. The X-ray tube may generate X-rays to a scanning object (usually a patient). The X-ray detector may receive X-rays that pass through the object and generate a reading corresponding to the received X-rays. The X-ray tube and the X-ray detector may be arranged opposite to each other on two ends of a C-shaped arm or a G-shaped arm. The C-shaped arm or G-shaped arm may be able to rotate freely in all three dimensions and hence X-ray images from any angle or slice of the object may be obtained. However, the distance between the X-ray tube and the X-ray detector (which is referred to as source image distance (SID)) is conventionally fixed. Some X-ray imaging apparatuses may have certain complex mechanical and electrical structures to adjust the SID, but the arm is usually tilted or rotated after the adjustment because the center of gravity of the arm or the ends thereof change. The tilt or rotation of the arm may cause a change of scanning areas and create discontinuity or artifacts in X-ray images. Thus, it may be desirable to provide an X-ray imaging apparatus of which SID can be easily changed without causing any tilt or rotation of the arm.